The Hindenburg Line was a vast system of defences in Northern France constructed by the Germans during the winter of 1916–17 during World War I; the Germans called it the Siegfried Line. It ran from the area around Arras to beyond St Quentin, and consisted of deep and wide trenches, thick belts of barbed wire, machine-gun positions, concrete bunkers, tunnels and command posts. The strategy behind the Line involved actually withdrawing from some territory on the front back towards the more easily defended ground of the Line; this shortened the length of the front by 25 miles and enabled the Germans to release 13 divisions for service in reserve.¹ This German withdrawal began in February 1917; the area between the old front and the Line was deliberately devastated as a scorched earth tactic. The Germans considered the Line virtually impregnable, but a successful Allied offensive in September 1918 proved otherwise.

Reference

1. Gilbert, Martin. The First World War (1994), chapter 16: "The intensification of the war".